Dopamine and Cu+/(2+) Can Induce Oligomerization of alpha-Synuclein in the Absence of Oxygen: Two Types of Oligomerization Mechanisms for alpha-Synuclein and Related Cell Toxicity Studies

Abstract

alpha-Synuclein oligomers can induce neurotoxicity and are implicated in Parkinson's disease etiology and disease progression. Many studies have reported alpha-synuclein oligomerization by dopamine (DA) and transition metal ions, but few studies provide insight into joint influences of DA and Cu2+. In this study, DA and Cu2+ were coadministered aerobically to measure alpha-synuclein oligomerization under these conditions. In the presence of oxygen, DA induced alpha-synuclein oligomerization in a dose-dependent manner. Cu+/2+ did not effect oligomerization in such a manner in the presence of DA. By electrophoresis, Cu2+ was found easily to induce oligomerization with DA. This implies that oligomerization invoked by DA is reversible in the presence of Cu2+, which appears to be mediated by noncovalent bond interactions. In the absence of oxygen, DA induced less oligomerization of alpha-synuclein, whereas DA/Cu2+ induced aerobic-level amounts of oligomers, suggesting that DA/Cu2+ induces oligomerization independent of oxygen concentration. Radical species were detected through electron paramagnetic resonance (EPR) spectroscopic analysis arising from coincubation of DA/Cu2+ with alpha-synuclein. Redox reactions induced by DA/Cu2+ were observed in multimer regions of alpha-synuclein oligomers through NBT assay. Cellular toxicity results confirm that, for normal and hypoxic conditions, copper or DA/Cu2+ can induce cell death, which may arise from copper redox chemistry. From these results, we propose that DA and DA/Cu2+ induce different mechanisms of alpha-synuclein oligomerization, cross-linking with noncovalent (or reversible covalent) bonding vs. likely radical-mediated covalent modification.